1. Field of the Invention
This invention relates to fiber lasers and more specifically to a single frequency 1 μm fiber laser with narrow linewidth (<10 kHz) formed from ytterbium doped oxide-based multicomponent glasses.
2. Description of the Related Art
Rare-earth doped glass fiber lasers were first proposed in the 1960s and have received considerable attention in the 1980s for potential applications in optical communication (Michel J. F. Digonnet, “Rare-Earth Doped Fiber Lasers and Amplifiers,” Marcel Dekker, New York 2001). For laser emission to occur, the active fiber is placed inside a resonant cavity. The optical feedback can be provided simply by the reflectivity of the end facets, by mirrors, by distributed feedback Bragg (DFB) gratings, or by distributed Bragg reflectors (DBR), or by constructing a ring cavity structure. Laser emission occurs when the total gain overcomes the losses in the cavity. Hence, a minimum gain has to be achieved to reach the laser threshold condition.
Typical fiber lasers lase in a great number of longitudinal modes in single transverse mode optic fibers, the so-called single mode fibers. A single longitudinal mode (single frequency) fiber laser 1.55 μm was demonstrated by Ball et al (G. A. Ball, W. W. Morey, W. H. Glenn, IEEE Photonics Technology Letters, Vol. 3, No. 7, July 1991) utilizing two intracore Bragg reflectors for cavity feedback and longitudinal mode discrimination in a 50 cm Er3+ doped silica fiber. U.S. Pat. No. 5,305,335 to Ball describes single frequency Er3+ doped fiber laser consisting of a pair of Bragg reflectors separated by a 9 cm cavity. U.S. Pat. No. 5,237,576 to DiGiovanni describes a fiber laser of 5 cm or less using fluorine-phosphorous-doped silica matched index cladding, a germania-alumina-doped silica outer core and an alumina-erbium-doped silica inner core in which the DBRs are formed in the doped fiber. W. H. Loh et al. Journal of Lightwave Technology, Vol. 16, No. 1, pp. 114–118 January 1998 describes a 1.5 cm phosphosilicate single-mode fiber lasers co-doped with Er:Yb. These lasers provide a 1.5 μm single-frequency output of less than 1 mW typically. A booster amplifier is required to produce output powers greater than 10 mW. U.S. Pat. No. 5,469,520 to Morey and U.S. Pat. No. 6,229,827 to Fernald describe tuning techniques using fiber Bragg gratings.
Other applications, such as, seeder lasers, LIDAR, optical heterodyne systems, nonlinear frequency conversion, coherent satellite communication, and distributed sensing require a robust 1 μm single frequency narrow linewidth fiber laser with output powers of at least 2 mW, preferably more than 20 mW. A 1 μm wavelength is required to match the gain energy level of the widely used Nd:YAG laser crystal at 1.064 μm. Single-frequency operation with a linewidth less than 10 kHz provides both the resolution and long coherence needed by these applications. The 1 μm laser should have sufficient output power without the need for a booster amplifier in order to maintain the high signal to noise ratio.
Current 1 μm single frequency narrow linewidth lasers are generated from solid state lasers using non-planar ring oscillators (NPROs) technology. U.S. Pat. No. 4,578,793 to Kane and U.S. Pat. No. 5,043,996 describe solid state monolithic nonplanar ring oscillators which can operate as unidirectional single-frequency lasers in the presence of a sufficiently strong magnetic field.
Current 1 μm fiber laser technology does not support such performance. Efficient single transverse mode 1 μm ytterbium silica fiber lasers are described by J. R. Armitage et al Electronics Letters, Vol. 25, No. 5, 1989, pp. 298–299 and H. M. Pask et al Electronics Letters, Vol 30, No. 11, 1994, pp. 863–864. M. Auerbach et al Optics Communications, 195(2001) pp. 437–441 reported a 1.6W 1 μm ytterbium fiber laser with a spectral line width of 600 MHz from a double-clad silica fiber. 0.5 mW Single-frequency 1 μm ytterbium-doped silica fiber laser was achieved using 1.5 to 10 m long silica fiber stabilized by spatial hole burning by R. Paschotta et al Optic Letters Vol. 22, No. 1, pp. 40–42. U.S. Pat. No. 5,710,786 describes a 1 μm pump source for pumping ionized praseodymium for amplification of signals in the 1280 to 1340 nm range. The pump source uses a silica-based glass double-clad optical fibre doped with triply ionized rare-earth ytterbium ions and fibre gratings formed therein. To efficiently pump the praseodymium, the pump source output has a single spatial (transverse) mode at the laser wavelength of 1012 to 1022 nm. U.S. Pat. No. 5,991,314 to Ionov et al describes cladding pumped ytterbium fiber laser for pumping erbium doped fiber amplifiers. All these ytterbium fiber lasers were demonstrated in silica fibers.
It is very difficult if not impossible to demonstrate high power single frequency narrow linewidth 1 μm ytterbium silica fiber laser with the present state of the art. It is therefore an object of the presentation invention to provide a high power single frequency narrow linewidth 1 μm fiber laser.